This paper explores qualitative changes in freshmen’s writing of factorial explanations throughout a genre-based syllabus implemented within a year in a Spanish L1 writing course offered at a Colombian university. The aim was to assess the extent to which explicit modelled and scaffolded writing of generic exemplars resulted in freshmen’s appropriation of the generic features of the target genre. The analysis of independently produced instances showed students’ appropriation of the contextual structure of factorial explanations as well as adequate control of textual and ideational resources for making meaning across generic stages and phases. We discuss findings in connection with the contribution of Genre Based Literacy Pedagogy in the development of reading and writing skills in initial literacy education at the university level.